Selective recovery from retrograde amnesia produced by hippocampal spreading depression.

Injections of potassium chloride into the hippocampus after learning produce temporary disruption of neural activity and retrograde amnesia. Recovery from the amnesia is selective-rats recover from amnesia of events that occurred24 hours before injection but do not recover from amnesia of events that occurred 10 seconds before injection.

Memory formation: evidence for a specific neurochemical system in the amygdala.

beta-Adrenergic antagonists injected into the amygdala complex of rats trained in a passive avoidance task produced time-dependent and dose-dependent decreases in retention of the task. In addition, the effects observed with beta-adrenergic antagonists were both stereospecific and reversed by norepinephrine. The results support a role for an amygdala beta-adrenergic system in memory processes.

Electrophysiology of the dorsolateral mesopontine reticular formation during Pavlovian conditioning in the rabbit.

Extracellular single-unit recording methods were used to study the activity of neurons within a restricted portion of the dorsolateral mesopontine reticular formation, an area which includes the parabrachial, pedunculopontine tegmental and cuneiform nuclei. Recordings were obtained during presentations of unfamiliar and familiar sensory stimuli, during Pavlovian differential conditioning procedures that elicited conditioned bradycardia, and while stimulating the amygdaloid central nucleus to identify neurons that projected to, or received projections from, the amygdaloid central nucleus. Activity in most dorsolateral mesopontine reticular neurons was altered during sensory stimulation, and the convergence of auditory and somatic inputs onto single neurons was common. Moreover, neural responses were often of a different magnitude and/or direction to auditory stimuli that were unfamiliar vs familiar vs reinforced (paired with pinna stimulation), and many of these differentially responsive neurons were activated orthodromically by stimulation of the amygdaloid central nucleus. In contrast, neurons activated antidromically by stimulation of the amygdaloid central nucleus were relatively quiescent during all phases of the experiment. Results are discussed in relation to current hypotheses concerning the functional significance of various neuronal subpopulations within the dorsolateral mesopontine reticular formation during Pavlovian conditioning.

Responses of amygdaloid central nucleus neurons to stimulation of the insular cortex in awake rabbits.

Recent evidence suggests that the amygdaloid central nucleus may contribute importantly to autonomic regulation during emotional states, possibly via both direct and indirect central nucleus efferent projections to autonomic regulatory nuclei in the lower brainstem. Additional findings suggest that the insular cortex may participate in autonomic regulatory processes, possibly by exerting an influence upon central nucleus neuronal activity via its direct projections to the central nucleus. The present experiment was conducted to determine the effects of insular cortex stimulation upon extracellularly recorded central nucleus neuronal activity in conscious, drug-free, rabbits. Satisfactory recordings were obtained from 146 central nucleus neurons. These were classified as belonging to one of six general categories on the basis of rates and patterns of ongoing discharge, responses to an auditory stimulus, and location within the nucleus. Determinations were then made as to whether each neuron could be activated antidromically from a ventrolateral mesencephalic region through which descending central nucleus projections to the lower brainstem course, and hence, whether the neuron might contribute to these projections. The activity of each neuron was then assessed during single-pulse stimulation of the region of the insular cortex demonstrated previously to project to the central nucleus. Such stimulation produced no response, an initial increase, or an initial decrease in the activity of 33%, 53% and 14% of the 146 neurons, respectively. The predominant response consisted of a single spike with a short but variable onset latency, suggesting orthodromic activation via one or a few synapses. Included among the neurons that responded to stimulation of the insular cortex in this manner were seven of 22 central nucleus neurons identified as projecting to the lower brainstem. Responses consisting of initially decreased activity most often occurred in neurons that discharged infrequently and were sensory-responsive. The results of this experiment thus provide more detailed information than was available previously regarding the pervasive influence of the insular cortex upon central nucleus neuronal activity, and provide further support for the notion that the insular cortex may participate in autonomic regulatory processes by way of its direct projections to the central nucleus.

Frontal cortex projections to the amygdaloid central nucleus in the rabbit.

Evidence has recently been presented which demonstrates that the amygdaloid central nucleus projects directly upon cardiovascular/autonomic regulatory nuclei of the dorsal medulla and that in the rabbit this nucleus may influence cardiovascular activity during emotional states. The present study is one of a series of investigations designed to provide information on the innervation of the central nucleus in the rabbit and describes the topography and origin of frontal cortex projections to the nucleus based upon retrograde and anterograde axonal transport techniques. Injections of horseradish peroxidase or the fluorescent dyes, Bisbenzimide or Nuclear Yellow, into the central nucleus resulted in abundant numbers of retrogradely labeled neurons in three regions of the frontal cortex: the insular cortex on the lateral surface and areas 25 and 32 on the medial surface of the hemisphere. The majority of labeled neurons in the insular cortex were located in layer V of the dorsal and posterior agranular insular regions, although labeled neurons were observed in layer V of the granular insular cortex as well as in layers II and III of the posterior agranular insular cortex. Labeled neurons in areas 25 and 32 were located throughout all layers and the total number of these neurons was substantially less than that observed in the insular cortex. Autoradiographic experiments in which amino acids were injected into the insular cortex resulted in a dense pattern of transported label within the central nucleus that extended rostrally into the sublenticular substantia innominata and lateral component of the bed nucleus of the stria terminalis. Label was also observed in the cortical, lateral, basolateral and basomedial amygdaloid nuclei. In contrast to the projections from the insular cortex, amino acid injections into areas 25 and 32 resulted in only relatively light labeling within the most rostral region of the central nucleus; otherwise the nucleus was partially encapsulated and virtually devoid of label. These results suggest that the insular cortex possesses the potential to directly influence the central nucleus projection to cardiovascular/autonomic regulatory nuclei of the dorsal medulla and thus, together with the amygdaloid central nucleus, appears to be an important component of a forebrain system involved in cardiovascular/autonomic regulation.

The organization of dorsal medullary projections to the central amygdaloid nucleus and parabrachial nuclei in the rabbit.

The amygdaloid central nucleus and the pontine parabrachial nucleus receive direct, ascending projections from autonomic regulatory nuclei of the dorsal medulla and are recognized as important components of a forebrain system which contributes to autonomic regulation. The present study was designed to provide more detailed information on the anatomical organization of this ascending system in the rabbit by determining (a) the extent to which separate populations of neurons within the solitary complex project to the central nucleus and parabrachial nucleus, (b) the topographical distribution of the projections of the solitary complex within the amygdaloid central nucleus and parabrachial nucleus and (c) the extent to which projections from the solitary complex to the parabrachial nucleus terminate in the region of origin of projections from the parabrachial nucleus to the amygdaloid central nucleus. A fluorescent dye, double retrograde-labeling technique demonstrated that separate populations of neurons in the solitary complex projected to the amygdaloid central nucleus and parabrachial nucleus. Neurons of both populations were more heavily concentrated within the caudal two thirds of nucleus of the solitary tract and were most numerous within the commissural, medial and dorsomedial subnuclei. Labeled neurons were also located within the dorsal motor nucleus of the vagus nerve. Autoradiographic experiments demonstrated that injections of amino acids into the solitary complex resulted in terminal labeling in the central nucleus. This labeling extended rostrally into the adjacent sublenticular substantia innominata and lateral component of the bed nucleus of the stria terminalis. Label was also observed within the lateral, medial, and Kolliker-Fuse regions of the parabrachial nucleus. A particularly dense field was observed overlying cells located within the ventrolateral region of the lateral parabrachial nucleus. This region contained the majority of labeled neurons within the parabrachial nucleus following fluorescent dye injections into the central nucleus. Furthermore, injections of amino acids into this region resulted in terminal labeling within the central nucleus, with a particularly dense area observed within the medial aspect of the nucleus. The results demonstrate that separate populations of neurons within the solitary complex of the rabbit project to the central amygdaloid and parabrachial nuclei and that the majority of these are located within the caudal two-thirds of the complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Contributions of the amygdaloid central nucleus to the modulation of the nictitating membrane reflex in the rabbit.

The contributions of the amygdaloid central nucleus (ACe) to the modulation of the amplitude of the nictitating membrane reflex (NMR) were determined. Experiment 1 demonstrated that low-level electrical stimulation of the ACe enhances the amplitude of the NMR when administered immediately preceding the elicitation of the reflex by an eyelid stimulus. In Experiment 2 the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase determined that the ACe projects to the entire rostrocaudal extent of the lateral tegmental field (LTF), the brainstem area in which the multisynaptic component of the unconditioned NMR pathway is believed to be located. These results are consistent with the hypothesis that the ACe, via its projections to the LTF, modulates reflex sensitivity during conditioned arousal and may contribute to the associative enhancement of the unconditioned NMR that occurs early during Pavlovian nictitating membrane conditioning.

Amygdaloid modulation of mesopontine peribrachial neuronal activity: implications for arousal.

This research sought to determine if activation of the amygdaloid central nucleus (ACe) modulates the activity of arousal-related neurons within the peribrachial (PB) region. Two categories of neurons were identified. Neurons of the 1st category demonstrated low spontaneous rates and responded with a burst of spikes to acoustic stimuli, characteristics similar to those reported for cholinergic ponto-geniculo-occipital (PGO) wave generator neurons. Neurons of the 2nd category demonstrated significant correlations between their spontaneous activity and the power of delta wave activity in the electroencephalogram (EEG) and responded to acoustic stimuli. Electrical stimulation of the ACe activated 43% of the PGO-related neurons but only 6% of the EEG-related neurons. The results suggest that the ACe modulates the activity of neurons that form the substrate for PGO waves, which are recognized correlates of arousal.

Response characteristics of neurons in the medial component of the medial geniculate nucleus during Pavlovian differential fear conditioning in rabbits.

Recent evidence suggests that the amygdaloid central nucleus (ACE) may contribute significantly to Pavlovian fear-conditioned bradycardic responses during the presentation of conditioned emotional stimuli. Because the medial component of the medial geniculate nucleus (MGm) is a major source of input to the region of the ACE, the extracellular single-unit responses of MGm neurons were examined during Pavlovian differentially conditioned bradycardic responding in rabbits. Conditioning involved pairing one tone (CS+) with paraorbital shock and presenting another tone (CS-) in the absence of shock. Two general classes of MGm neurons were identified based on their conditioned-response characteristics. Both groups responded differentially to the CSs. One group responded with greater increases in activity and at a shorter latency to the CS+ compared with the CS-, whereas the other group responded with greater increases in activity and at a shorter latency to the CS- compared with the CS+. Recordings from MGm neurons in naive rabbits prior to conditioning provided evidence that the acoustic stimuli used subsequently as the CS+ and CS- did not evoke differential responses. These results suggest that the MGm along with the ACE may be forebrain components of a neural circuit involved in the acquisition and/or expression of Pavlovian fear-conditioned bradycardic responses.

Effects of electrical stimulation of the amygdaloid central nucleus on neocortical arousal in the rabbit.

This study sought to determine whether electrical stimulation of the amygdaloid central nucleus (ACe) produces cholinergically mediated neocortical arousal manifested in the suppression of frontal cortex delta wave (1-4 Hz) activity. Stimulation in both anesthetized and conscious rabbits produced a suppression of delta activity that was accompanied by bradycardia and blocked by cholinergic antagonists. Stimulation of the adjacent putamen did not produce delta suppression, whereas stimulation of the adjacent ventral globus pallidus produced a suppression of shorter duration than that produced by ACe stimulation. The results suggest that the ACe influences neocortical arousal, which may be mediated by its influence on the activity of cholinergic neurons of the nucleus basalis.

Arousal-related associative response characteristics of dorsal lateral geniculate nucleus neurons during acoustic Pavlovian fear conditioning.

This research determined whether fear-conditioned, acoustic stimuli induce thalamic arousal reflected in associative responses in dorsal lateral geniculate nucleus (dLGN) neurons. Rabbits received a Pavlovian discriminative fear conditioning procedure in which one tone conditioned stimulus (CS +) was always paired with an aversive unconditioned stimulus (US) and another tone (CS-) was never paired with the US. Responses of single dLGN neurons to random CS+ and CS- presentations were then recorded. Nine of 15 recorded neurons demonstrated significantly greater firing during the CS+ versus the CS-. Their spontaneous activity demonstrated tonic firing during increased neocortical arousal and burst firing during decreased neocortical arousal. The results demonstrate that dLGN neurons show associative responses to fear-conditioned, acoustic stimuli and present a model for investigating the neural circuits by which such stimuli affect sensory processing at the thalamic level.

The effects of intra-amygdaloid infusions of a D2 dopamine receptor antagonist on Pavlovian fear conditioning.

The present study examined the effects of bilateral intra-amygdaloid infusions of the D2 receptor antagonist, eticlopride, on the acquisition and expression of Pavlovian fear conditioning as measured by freezing to acoustic and background contextual stimuli in the rat. Infusions of eticlopride before acquisition or before both acquisition and retention testing significantly attenuated conditioned freezing to tone presentations during the retention test 24 hr later. No effects, however, were observed on freezing that emerged during acquisition. Furthermore, these effects were not attributable to state-dependent learning effects or alterations in baseline activity or shock reactivity. In conclusion, these results suggest that amygdaloid dopamine transmission at D2 receptors contributes to the formation and/or consolidation of fear memories.

Benzodiazepine effects on heart rate conditioning in the rabbit.

The present experiment was undertaken to assess the ways in which benzodiazepine administration alters heart rate responding during Pavlovian aversive conditioning in the rabbit. Each of three benzodiazepine compounds (chlordiazepoxide, flurazepam, diazepam) reliably attenuated the magnitude of the conditioned bradycardia response as compared to vehicle controls. Lower doses of two of these compounds significantly potentiated the conditioned bradycardia response. Benzodiazepine treatment did not significantly alter baseline heart rate, the expression and habituation of the heart rate orienting response, or heart rate responding during unpaired stimulus presentations. The effects of benzodiazepines under these conditions therefore appeared to be selective to those heart rate responses that were conditioned. These results are consistent with evidence suggesting the involvement of benzodiazepine-sensitive processes in the expression of emotional responses, including concomitant cardiovascular alterations.

Purkinje cell responses in the anterior cerebellar vermis during Pavlovian fear conditioning in the rabbit.

Extracellular single-unit recordings of Purkinje cells in the anterior cerebellar vermis (ACV) of the rabbit found evidence of short-latency (20-30 ms) differential responses to discriminatively-conditioned auditory stimuli during Pavlovian fear conditioning procedures. These differential unit responses appeared to be a function of learning as differential ACV Purkinje cell responses were not observed in naive (untrained) animals. Some of these evoked neurophysiological responses were also correlated with the behavioral conditioned autonomic response. (CAR); a conditioned bradycardiac response. These electrophysiological data, coupled with previous lesion results, suggest that the ACV is part of an important neural circuit for Pavlovian conditioned bradycardia.

Peptide-containing pathways from the parabrachial complex to the central nucleus of the amygdala.

The present investigation was undertaken to examine the organization of peptidergic projections that exist between the parabrachial nuclear complex (PB) and the central nucleus of the amygdala (CNA). The retrograde tracer True Blue was injected into the CNA of adult rats. The brain tissue was then reacted immunocytochemically to localize neurotensin (NT), substance P (SP), methionine enkephalin (ENK), vasoactive intestinal polypeptide (VIP), somatostatin (SS), and cholecystokinin octapeptide (CCK). Following microinjection of True Blue in the CNA, retrogradely-labeled neurons were located primarily in the external lateral subnucleus, abutting the brachium conjunctivum. In animals that received colchicine pretreatment, two populations of neurons, containing either SP or NT, were found to project to the CNA. In addition, cells containing CCK, ENK, VIP, or SS were not found to be a part of this projection system. These data suggest that neurons in the PB project to the CNA and are, in part peptide-containing.

An electrophysiological characterization of ventral tegmental area dopaminergic neurons during differential pavlovian fear conditioning in the awake rabbit.

Recent research has suggested that the mesencephalic dopaminergic (DA) system is activated by stress. For example, alterations in DA metabolites have been found in the ventral tegmental area (VTA) following footshock and immobilization in the rat [15,37]. Furthermore, this activation appears selective to DA neurons within the VTA since no changes were observed within the substantia nigra [15,16]. While this research suggests that DA neurons in the VTA are activated by aversive events, there has been a paucity of electrophysiological research designed to examine the sensory response characteristics of these DA neurons, and in particular their response to stimuli which predict aversive events. The present study was conducted to investigate the response characteristics of DA neurons within the VTA of the awake rabbit to acoustic stimuli which, via Pavlovian aversive conditioning procedures, came to predict the occurrence of a mild shock to the pinna. 45%, of the neurons meeting pre-established criteria for DA neurons demonstrated either significant excitation or inhibition to conditioned aversive stimuli. These neurons responded differentially to CS+ and CS- presentations. Some of these neurons (65%) demonstrated a greater increase in activity during the CS+ compared to the CS-, some (22%,) demonstrated a greater decrease in activity during the CS+ compared to the CS- and some (13%) demonstrated a greater increase in activity during the CS- compared to the CS+. Further, conditioned heart rate responses in the rabbits occurred during the recording of a majority of these neurons. These overall results suggest that conditioned aversive stimuli can affect the firing of VTA DA neurons and that these neurons comprise a heterogenous population with respect to their response profiles.

Electrophysiological characteristics of amygdaloid central nucleus neurons during Pavlovian fear conditioning in the rabbit.

Recent evidence suggests that the amygdaloid central nucleus (ACE) may contribute importantly to cardiovascular adjustments in response to the presentation of conditioned emotional stimuli, possibly via direct ACE projections to cardiovascular regulatory nuclei in the medulla. The present experiment was conducted to obtain additional data relevant to this suggestion. Extracellular single-unit recordings were obtained from 85 histologically-verified ACE neurons during Pavlovian differentially conditioned heart-rate responding in rabbits. Conditioning involved pairing one tone (CS+), but not a second tone (CS-), with paraorbital shock. Those ACE neurons which project to the lower brainstem were identified by their antidromic responses to stimulation of a mesencephalic region through which descending ACE projections course. Under these conditions it was possible to classify ACE neurons as conforming to one of 6 general categories based on their spontaneous activity and conditioned response characteristics. In addition, it was determined that: (1) the electrophysiological characteristics of many ACE neurons were differentially altered in response to presentations of the CS+ versus the CS-; (2) the responses of many ACE neurons to presentations of the CS+ were correlated with the magnitudes of concomitant conditioned alterations in heart rate; and (3) the activity of antidromically-identified ACE neurons which project to the lower brainstem was decreased in response to presentations of each CS. These data provide additional support for the notion that the ACE contributes to cardiovascular regulation during the presentation of emotionally-arousing stimuli.

Amygdaloid D1 dopamine receptor involvement in Pavlovian fear conditioning.

The amygdala has long been implicated in conditioned fear. The mesencephalic dopaminergic system provides a rich innervation to the amygdala [J.H. Fallon, P. Ciofi, Distribution of monoamines within the amygdala, in: J.P. Aggleton (Ed.), The Amygdala: Neurobiological Aspects of Emotion, Memory and Mental Dysfunction, Wiley, New York, 1992, pp. 97-114; L.J. Freedman, M.D. Cassell, Distribution of dopaminergic fibers in the central division of the extended amygdala of the rat. Brain Research 633 (1994) 243-252; E. Asan, The catecholaminergic innervation of the rat amygdala. Advances in Anatomy Embryology and Cell Biology 142 (1996) 1-107]. Specific activation of the mesoamygdaloid dopaminergic system has been reported to occur in response to conditioned fear-arousing stimuli [M.L. Coco, C.M. Kuhn, T.D. Ely, C.D. Kilts, Selective activation of mesoamygdaloid dopamine neurons by conditioned stress: attenuation by diazepam. Brain Research 590 (1992) 39-47] suggesting that dopamine release in the amygdala may contribute to the acquisition and/or expression of conditioned fear. Using a 2x2 factorial design, Experiment 1A investigated the effects of bilateral intra-amygdaloid infusions of the selective D1 receptor antagonist, SCH 23390 (2.0 microgram 0.5 microliter-1 side-1), on the acquisition and expression of Pavlovian conditioned fear measured by freezing to acoustic and background contextual stimuli. Infusions of SCH 23390 prior to acquisition training, prior to retention testing or prior to both significantly attenuated conditioned freezing during retention testing. Experiment 1B investigated the dose-dependent effects of pre-training infusions of SCH 23390 (0.5, 1.0 and 2.0 microgram) on conditioned fear. Pre-training infusions of SCH 23390 dose-dependently attenuated conditioned freezing during retention testing. Experiment 2A investigated the effects of bilateral infusions of the selective D1 receptor agonist, SKF 82958 (2.0 microgram 0.5 microliter-1 side-1) on the acquisition and expression of conditioned fear. Infusions of SKF 82958 prior to training facilitated conditioned freezing during retention testing. Experiment 2B investigated the dose-dependent effects of pre-training infusions of SKF 82958 (1.0, 2.0 and 4.0 microgram) on conditioned fear. Pre-training infusions of SKF 82958 dose-dependently facilitated conditioned freezing during retention testing. In conclusion, these results suggest that dopamine transmission within the amygdala contributes to the acquisition and expression of Pavlovian fear conditioning.

The organization of insular cortex projections to the amygdaloid central nucleus and autonomic regulatory nuclei of the dorsal medulla.

Using the fluorescent dye, double retrograde-labeling tracing technique, separate populations of insular cortex neurons were demonstrated to project to the amygdaloid central nucleus and to autonomic nuclei of the dorsal medulla. Both populations were located in layer V of the agranular and granular insula with neurons projecting to the dorsal medulla demonstrating a more medial distribution. The results yield additional detail on the organization of forebrain areas involved in autonomic regulation.

Multiple unit activity recorded from amygdala central nucleus during Pavlovian heart rate conditioning in rabbit.

Using a Pavlovian heart rate conditioning paradigm, a rapid development of short latency increases in the multiple unit activity of the amygdala central nucleus were observed in response to a tone conditioned stimulus. In some cases the increase in multiple unit response showed a parallel development with the conditioned decelerative heart rate response and were significantly correlated with it. These results suggest a direct role for the central nucleus in the expression of conditioned heart rate responding in rabbit.